Such a circuit arrangement and a related method are known from DE 10 2007 057 772 A1.
A general problem during operation of discharge lamps is the changes in the electrode geometry over the course of their life. This applies in particular to the frontmost region of the electrode head, where, as a result of the arc attachment, temperatures close to the melting point of the electrode occur. In the case of lamps operated on alternating current, in particular in the case of lamps which are used in video projection, the growth of tips on the electrode head can be achieved by suitable operational parameters. Such tips have a positive effect on the properties of the lamp, for example in respect of luminance and electrode burnback. The response over the life and the effective luminous flux of such a lamp are therefore critically dependent on the stability of the electrodes or the electrode tips that have grown on over the course of the life. Of particular relevance in this case are the length and the diameter of the electrode tips.
Depending on specific conditions, generally the following response can be observed: in the case of excessive burnback, the electrode tips become small and narrow. Excessive fusing, on the other hand, results in the electrode tips becoming very wide or long. Moreover, this may result in an asymmetric development of the electrode tips.
In the related art, there is a large number of documents which handle the topic of electrode stability, in particular in respect of excessive electrode burnback, on the one hand, or in respect of excessively pronounced fusing of the electrodes, on the other hand. In this context, reference is made to WO 2009/007914 A1, for example.
The starting point with the method known from the related art is generally an apparatus with the aid of which a value is determined which represents a measure of the present length of the electrode spacing. Generally, the measurement of the lamp voltage with the aid of a suitable circuit which is integrated in the electronic ballast for operation of the lamp is intended thereby. Depending on the measured value of the lamp voltage, in the case of one or more voltage threshold values changes to the operational parameters of the discharge lamp are performed, for example matching of the lamp frequency or the profile of the lamp current.
One disadvantage with these known methods consists in that an asymmetric development of the electrode tips is not detected. Moreover, the absolute value of the voltage is only correlated conditionally with the state of the electrode tips which is actually of interest, i.e. in the case of two lamps with one and the same voltage value, the state of the electrodes can differ markedly, for example determined by manufacturing tolerances during lamp construction, but also in terms of the application used by the customer.
This is improved by the teaching of DE 10 2007 057 772 A1, already mentioned, which discloses the circuit arrangement of the generic type or the method of the generic type.
Said document is concerned with the avoidance of flicker phenomena and of the reduction of the lamp voltage in the case of excessive formation of electrode tips. In order to prevent these effects, the document proposes suppressing commutation operations during operation of the discharge lamps with a square-wave current, as a result of which fusing of the electrode tips arises. In order to detect the tip geometry, it is proposed in particular to suppress the switching during a first test time in a first polarity and in the process to determine the change in the lamp voltage, and then to suppress the switching during a second test time of the same duration as the first test time in a second polarity, which is different than the first polarity, and in the process again to determine the change in the lamp voltage. Finally, the switching is suppressed during a fusing time, which is longer than the test times, wherein, during the fusing time, the polarity which effected the greater change in the lamp voltage during the preceding test times is selected.
US 2006/0012309 A1 discloses a method in which attempts are made, by suitable operational parameters, to compensate for asymmetries which are expected from the beginning during the life. US 2010/0052496 A1 discloses a method in which electrodes dimensioned differently from the beginning are used in order to compensate an expected asymmetry.
In relation to further related art, reference is also made to WO 2010/086222 A1.
The disadvantage of the procedure known from the mentioned DE 10 2007 057 772 A1 consists in that this procedure sometimes results in good results, but often also in unusable results.